Apparatus for charging fibrous material



Oct. 20, 1970 f2. WQCROOK my 3,535,538 APPARATUS FOR CHARGING FIBROUSMATERIAL 2 Sheets-Sheet 1 Filed April 6,; 1967? ATTORNEY 20, 1970 as. W.CHUOK m 3,535,588

APPARATUS FOR CHARGING FIBROUS MATERIAL Filed April 6, 1967 v 2Sheets-Sheet 2 INVENTOR FHPP 4944x705 owe/(E;

ATTORNEY nited States Patent 3,535,588 APPARATUS FOR CHARGING FIBROUSMATERIAL Rapp Wallace Crook 11], Newark, Del., assignor to E. L

du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware Filed Apr. 6, 1967, Ser. No. 628,983 Int. Cl.H011 3/04 US. Cl. 317-4 7 Claims ABSTRACT OF THE DISCLOSURE A coronacharging device consisting of an ion gun and a grounded targetelectrode. The ion gun includes a plurality of corona generating points,each being connected in series through a resistor to a high voltagesource. A conductive sheath connected to the high voltage sourceencloses the resistors and connections to the corona gencrating pointsto protect the insulating materials in the ion gun from degradation dueto high voltage environment.

BACKGROUND OF THE INVENTION This invention concerns an apparatus forapplying a uniform electrostatic charge to a wide swath of movingfibrous material.

In the Steuber patent US. 3,169,899 a process is described for making anon-woven sheet from flash-spun fibrous material. In the flash spinningtechnique, a solution of an organic polymer, which is under pressure andat a temperature far above the boiling point of the solvent is extrudedinto an area of substantially atmospheric pressure. As the materialissues from the orifice, the solvent expands rapidly and aplexifilamentary strand is formed. The plexifilamentary strand iscomposed of very thin filmfibril elements which are interconnected in athree-dimensional network as described in detail in Blades and White,US. Pat. 3,081,519. The three-dimensional network is preferably spreadinto a wide web by causing it to impinge on a curved surface whereuponthe expanding solvent gas spreads the material. The control of thecollection of this network by aerodynamic and electrostatic means hasbeen the subject of much research. The problem is particularly diflicultbecause of the extremely fine I nature of the fibrils and because of thehigh volume and high velocity of the gas in the system.

Electrostatic charging of the fibers has been used to promote mutualrepulsion (better dispersion) of fibers and to promote better adherenceof fibers to a collecting roll or belt. In the Steuber patent, forexample, a rakelike electrode is shown. While this device improves thetendency of the fiber to be attracted to a grounded or oppositelycharged collection surface, it has deficiencies particularly withrespect to uniform charging and deposit of the web.

Some of the problems in uniformity have been eliminated by use of aparticular multi-needled electrode described in copending US.application Ser. No. 372,623 of Hollberg and Owens, filed June 4, 1964,now US. Pat. No. 3,387,326; wherein each needle is connected in seriesto a high impedance resistor which is, in turn, connected to a source ofhigh voltage direct current power. While this type of apparatus greatlyimproves the control of the laydown process, the life of the apparatusis less than desirable for commercial operation. The resistors in thissystem are located remotely from the spinning area to provide a lowcross-sectional area in the gas stream and the lowest possibleresistance to flow of gases. Unfortunately, the relatively fine wireswhich connect the needles of the ion gun to the resistors are themselvessources of corona discharge, especially whenever breakdown of theinsulating materials occur because of solvent or high voltage attack.Likewise, the connecting terminals between the wires and the needles orbetween the wires and the resistors are sources for corona discharge. Asa result, the insulating materials are degraded, and with thedegradation increasing exposure to corona discharge occurs. Eventually,the insulating materials become inefi'ective, excessive power lossesoccur, and the ion gun must be replaced.

SUMMARY OF THE INVENTION The purpose of the present invention is toprovide an ion gun which has low resistance to gas flow, providesuniform charging across a wide web of fibrous materials in a gaseousstream and has low susceptibility to degradation by a high voltageenvironment, or solvents.

The apparatus of the present invention is an ion gun for use inconjunction with a grounded target electrode to apply a uniformelectrostatic charge to a web of moving fibrous material advancingbetween the ion gun and the target electrode. The ion gun comprises aconductive tubular housing with ports substantially equi-spaced alongits length and with a conductive resistor enclosure joined to at leastone end of the tubular housing. The ports along the length of thetubular housing are provided with individual conducting needles whichextend from the tube. Needles are sealed in the ports and areelectrically insulated from the tubular housing. Each needle isconnected through an insulated conductor of small diameter to oneterminal of an insulated resistor located in the conductive resistorenclosure. Means are provided to connect the tubular housing, theresistor enclosure and the second terminal of each resistor to a commonsource of high voltage direct current power to provide a charged sheathenclosing the connecting means and the resistors.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustrationindicating the arrangement of various elements in a system wherein thecorona charging device of this invention is useful.

FIG. 2 is a front elevation of the U-shaped ion gun shown in FIG. 1.

FIG. 3 is an enlarged partially sectioned fragmentary view of FIG. 1,showing the relationship of the conducting needles of the ion gun to thetarget electrode.

FIG. 4 is a partially sectioned front view of the resistor housing.

FIG. 5 is a cross section through the ion gun shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 a spinneret10 is connected to a source of polymer dissolved in an organic solvent.Polymer solution 12 is extruded through orifice 14 in spinneret 10 intochamber 16 which is substantially at atmospheric pressure. Due to thepressure drop at spinning orifice 14 the solvent expands greatly and iscarried around the curved surface of a lobed deflector 18 onto thesurface of an annular target electrode 20. From the outer circular edgeof the target electrode the solvent gas and the film fibril material 22are carried downward in a linear path toward a moving collection surfacenot shown. Deflector 18 is connected to motor 24 which continuouslyrotates the deflector and thereby imparts oscillatory movement to thenetwork of film-fibril material as it is deflected from the lobedsurface. Annular target electrode 20' is coupled, for rotary movementabout deflector 18, to motor 26 through ring 28 and pinion gear 30attached to the shaft of motor 26. Target electrode 20 is connected toground through a contracting carbon brush 32, lead 34 and microammeter36. An ion gun 3-8 is positioned on the opposite side of the path ofadvance of the film fibril material 22 from the target electrode and isconnected to a high voltage source of direct current (DC) power 40through lead 42. Preferably, a negative D.C. source in the range of from45 to 70 kilovolts is used. The electrical potential which is providedbetween ion gun 38 and target elec trode 20 ionizes the solvent gasbetween the two and the ionized gas molecules attach themselves ordeposit a charge on the moving fibrous material 22.

Referring now to FIGS. 2-5, the arrangement of a U-shaped ion gun 38opposite annular target electrode 20 with the deflector 18 centeredwithin electrode 20 is shown in FIG. 2. In the U-shaped embodiment, theion gun 38 includes a conductive tubular housing 44 with numerous ports46 along its length. Connected to each end of housing 44 is a conductiveresistor enclosure 48. Housing 44 and enclosure 48 are joined in such amanner to form a good electrical connection between them, therebyproviding a conductive sheath surrounding the internal components of iongun 38. The entire ion gun structure is supported by support arms 50which are composed of electrical insulating material and are cemented tohousing 44. The lower portion of U-shaped housing 44 is generally ovalin cross section (FIGS. 3 and and fabrication is accomplished byselecting a straight length of tubing, e.g., aluminum, flattening aportion of the tubing to the desired oval shape then bending theflattened portion into a U-shape.

The oval shape may be obtained by inserting a flat bar havingrounded-edges into the undeformed tube then hammering or pressingportions of the tube so as to deform it to the desired cross-section.The step of bending the tube into a U-shape is best carried out bytemporarily filling the entire interior of the tube with a soft, easilybent metal alloy having a very low melting point such as Cerrobend, bycarrying out the bending operation on a conventional three-roll formeror in a press and, finally, by heating the tube to melt and remove thetemporary metal filling. In the lower portion of the tubular housing 44,spaced from each other with a chordal distance of about 0.965 cm. (or710 angular spacing), are holes 46 (FIG. 2). As best shown in FIG. 5each hole 46 is occupied by an insulative tubular insert 52 in the outerend of which is a close-fitting insulating jacket 54 and in the innerend of which is a close-fitting conductive metal bushing 56, whichcarries a screw thread on its interior surface. Extending through thecenter of the insulating jacket 54 is a corona generating needle 58which is threadedly engaged in the metal bushing 56. Engaged in theother end of bushing 56 is a round head screw 60 to which is soldered awire 62. Inserts 52 are, e.g., Teflon (registered trademark)fluorocarbon resin. The surface of the inserts (inside ion gun) has beenchemically etched to allow adhesion to epoxy. The entire inside of thetubular housing 44 is potted with an insulative resin 64 which holdsinserts in position. The axes of the needles 58 are generally at a rightangle to a plane running through the left-hand face of the housing 44 aswell as perpendicular to the planar surface of the target electrode(FIG. 3). The needles 58 are adjusted, so that their sharp, conicalpoints are equidistant from the surface of the target electrode 20 beingspaced therefrom by about 1.3 cm. As shown in FIG. 5 a portion of theexterior surface of the housing 44 is coated with a thin layer 66 ofepoxy resin, an insulating material used to reduce the tendency forcorona discharge to occur between the needles 58 and the outer surfaceof the sheath 44.

Each wire 62 connects a needle 58 and a resistor 68 and is electricallyinsulated along its length from the corona generating point to theresistor. The wires 62 pass upwardly through the legs of housing 44 intothe confines of one of the aluminumresistor enclosures 48 where eachwire 62 is joined to one terminal of its respective resistor 68 (FIGS. 4and 5), the resistance of these being substantially equal and being inthe range of 1 to 1000 megohm. The ends of the wires 62 are insulatedfrom each other where attached to the resistors 68 by means ofinsulating perforated partition 70, connected to enclosure 48. Theresistors are also protected by insulative materials to prevent shortingto resistor enclosures 48 or to one another; the opposite ends of theresistors are joined to a brass bus bar 72, which along with partitions70 supports the resistors in enclosure 48. The bus bar 72 isconductively secured to enclosure 48 which is provided with a threadedhole to receive a stud 74 connected to lead 42 which is in turnconnected to DC. source 40. Wires 62 are preferably pro-coated withinsulating materials and may be further insulated by pouring insulativeresins 64 or other insulating materials into the tubular housing 44While the wires are held in position. In a completely sealed ion gun theresistor housings 48 as well as tubular housing 44 are filled withresins. A resin particularly suited for this use is Dow CorningDielectric Gel.

It can be readily seen from the above described structure that a chargeequivalent to the DC. source 40 (45- 70 kilovolts) is placed on resistorhousing 48 and tubular housing 44 by means of the electrical continuitybetween source 40, lead 42, stud 74 and housings 48 and 44, thusproviding a charged sheath around the internal components of ion gun.38. The needles 58 are also connected to DC. source 40 through leads62, resistors 68, bus bar 72, housing 48, stud 74 and lead 42. Theneedles are charged to a somewhat lower voltage, this beingequivalent tothe DC. source potential of from about 45 to 70 kilovolts minus thevoltage drop in the resistors.

It can be readily appeciated that the charge placed on housings 44, 48electrically shields the components inside these housings from groundand greatly reduces the voltage stress placed on these components whichin turn minimizes degradation of insulative materials within thehousings resulting in extended life for the ion gun.

-In operation of the charging apparatus, a corona current at each needle58 from about 5 to about 30 microamps is adequate, with between 10 and20 microamps per needle being preferred for charging a film-fibrilnetwork. At these low currents of 5 to 30 microamperes per point and inthe absence of the resistors 68 the effect of fluctuations in thedynamic resistance between electrodes would be magnified. The ion gundescribed herein, however, provides a high impedance circuit to eachpoint so that normal fluctuations in the eifective dynamic resistance ofcorona discharge have little effect on emitted current. This is done byusing a resistance 68 of sufficient magnitude in series with each needle58 to provide a voltage drop of at least about 3,000 volts. In a typicalion gun target configuration the effective dynamic resistance of coronadischarge is about 60 megohms, whereas the resistance placed in serieswith each needle 58 to provide a corona current of at least 5 microampsis typically 600 megohms and for 12 to 20 microamps is typically 270megohms. Use of the resistors 68 makes the needle-to-needle currentvariations much less sensitive to such factors as point/ target spacing.

The position of corona generating needles 58 with reference to targetelectrode 20 is important for eflicient operation. It will be apparentthat the clearance between the needles and the target electrode shouldbe as small as efficient operation will permit. Generally, a clearanceof from about 1 cm. to about 2.5 cm. is satisfactory although this willvary with the design and capacity of the particular equipment.

It should be noted that as the distance between the needle points andthe target electrode is reduced, the total applied voltage across theresistor and the gap should be reduced to keep the current per point inthe 5-30 microampere range. In addition, as the needle pointto-electrodedistance is decreased it is desirable to in crease the number of pointsper inch in order to deposit a uniform charge on all parts of the web offibrous elements, provided the diameter of the inserts 52 is maintainedlarge enough to prevent discharge from the neodles to the conductivesheath or an insulating layer 66 is applied to the exterior of housing44 (FIG. 5). On the other hand, when increasing the distance between theneedle points and the target electrode the voltage should be increasedand/or the number of points per cm. decreased. For optimum operation,the points should be equally spaced along the tubular housing 44. It hasbeen found convenient in developing dimensions for the ion gun andtarget electrode to create a carbon black deposit on target electrode 20by spraying powdered carbon black into the operating area between theplate and the gun. An oval pattern is outlined by carbon depositsopposite each needle indicating the area of electrostatic influence ofeach needle under the particular conditions employed. The pattern laiddown by single points are centered the same distance apart as theneedles. The shape and size of the oval varies depending on needle totarget spacing.

Making the circle diameter for the needles'smaller (at the same currentlevel and same target plate diameter) results in pinning or clinging ofthe web to the target electrode. This results in bunching for aninstant, an uneven discharge across the web width, and a falling free ofthe bunched web to give a non-uniform sheet. On the other hand, if theion gun is aimed too near the outer edge of the target electrode,secondary ionization will develop at the edge of the target electrode 20generating ions of opposite polarity which will tend to discharge theweb. Thin layer 21 of epoxy resin at the outer peripheral edge of targetelectrode 20 (FIG. 3) is useful in reducing secondary ionization at theedge of the target electrode by eliminating a sharp conductive edge.Smooth operation of this equipment with uniform laydown on thecollection surface was readily obtained. In the optimum arrangement theoutside diameter of the target electrode 20 was 19.2 cm., the insidediameter of the epoxy layer 21 on the face of the target electrode was18.4 cm. and the diameter of the circular ring of needles 58 in the iongun was 16.5 cm.

Considering again FIGS. 1 and 3 the flow of solvent gases tends toaspirate additional gases indicated by the arrows over the tubularportion 44' of the ion gun 38 on the side nearest the spinneret It is tobe noted that the inner surface 45 (FIG. 3) of the ion gun is conicaland generally concentric with the conical surface 47 of the spinneret10. Thus there is smooth flow of the atmospheric gases over the U-shapedion gun into the flow line provided by the solvent escaping from theorifice 14. In a similar manner, gas is aspirated along the bottomsurface of the target electrode and is carried toward and past the backsurface of the thin outer epoxy-covered edge 21 of target electrode 20.Because of the aerodynamic design, turbulence is minimized and theformation of the low pressure area along the face of the targetelectrode 20 is maximized. This permits the fibrous web 22 to ride closeclose to the target electrode surface in a zone of high voltage gradientand maximum charging efficiency.

This invention has been described in detail with reference to a U-shapedapparatus, other shapes are equally suitable. For example, the tubularhousing of the apparatus may be perfectly straight. Likewise it may haveresistor housings on either one or both ends. However, it is obviousthat the shape of the ion gun should conform to the shape of the targetelectrode edge, since the gun should be aimed close to the outer edge ofthe target electrode, e.g., in the U-shaped embodiment the gun ispositioned so that the outer circular edge of the target electrode isconcentric with the curved portion of the U- shaped housing of the gun.

An important consideration in the design of the tubular part of theapparatus is the need for maintaining minimum resistance to flow of theflash-spinning solvent and of the entrained atmosphere. This isfacilitated by the concentricity between the proximal surfaces of thespinneret and the tubular housing of the ion gun and the remote locationof the resistor enclosures 48 relative to the path of the gas streamflowing over the lower part of the U- shaped tube.

While the ion gun has been shown in connection with flash spinning, itobviously is useful with other modes of spinning such as, melt spinningand dry spinning.

It is apparent that many modifications may be made in the disclosedapparatus without departing from the spirit of my invention which is,therefore, intended to be limited only by the scope of the appendedclaims.

What is claimed is:

1. In an apparatus for applying an electrostatic charge to fibrouselements forwarded along a path, an ion gun comprising:

(a) a plurality of needles in fixed parallel spacial relationshipdisposed across one side of said path;

(b) a direct current source of high voltage power;

(c) means connecting each needle to said source of high voltage power;and

(d) a conductive sheath connected to said high voltage source, saidsheath enclosing and reducing the voltage stress on said connectingmeans.

2. The apparatus of claim 1 wherein each connecting means includes ahigh impedance resistor.

3. The apparatus of claim 2 wherein said conductive sheath includes aU-shaped tubular housing and an enclosure joined to at least one end ofthe tubular housing, said enclosure being connected to said high voltagesource, said resistors being mounted in said enclosure and having oneend connected thereto, the other end of said resistors being connectedto said needles, said needles projecting from said tubular housing andbeing electrically insulated therefrom.

4. An apparatus for applying an electrostatic charge to fibrous elementsforwarded along a path, said apparatus comprising:

(a) a grounded target electrode disposed on one side of said path;

(b) a plurality of needles in fixed parallel spacial relationshipdisposed completely across the opposite side of said path, said needleshaving points directed toward said target electrode;

(c) a direct current source of high voltage power;

(d) means connecting each needle to said source of high voltage power,each connecting means including a high impedance resistor; and

(e) a conductive sheath connected to said high voltage source, saidsheath enclosing and reducing the voltage stress on said connectingmeans.

5. The apparatus of claim 4 wherein said conductive sheath includes aU-shaped tubular housing and an enclosure joined to each end of thetubular housing, the interiors of said enclosure and said tubularhousing being in communication, said enclosure being connected to said'high voltage source, said resistors being mounted in said enclosure andhaving one end connected thereto, the other end of said resistors beinginsulated from said enclosure and said housing and being connected tosaid needles, said needles projecting from said tubular housing andbeing electrically insulated therefrom.

6. The apparatus of claim 5, said target electrode having an outercircular edge concentric with the curved portion of the U-shapedhousing.

7. The apparatus of claim 5, said tubular U-shaped housing and saidenclosure being filled with an insulative resin.

References Cited UNITED STATES PATENTS 3,387,326 6/1968 Hollberg et al264-24 X J D MILLER, Primary Examiner A. D. PELL I-NEN, AssistantExaminer U.S. Cl. X.R. 18-8; 317-262

